This invention relates generally to nuclear reactor systems and more particularly, to a liquid metal cooled nuclear reactor arrangement which facilitates both through the head instrumentation and refueling of assemblies from in the nuclear core.
In any nuclear reactor which is intended to operate over an extended period of time, such as a reactor which is to be utilized in the generation of electrical power, a reactor facility should be constructed for removal and disposal of used or "spent" fuel element and for recharging or refueling of the reactor with new or unused fuel element. This of necessity involves providing access to the core of the nuclear reactor, this access usually being provided by an opening in the reactor vessel heads.
With liquid metal cooled fast breader reactors, it is desirable that only a small opening in the vessel head be exposed at any one time to reduce the size of temporary shield and shielding required to prevent the release of fission gases, contact of a liquid metal coolant, such as sodium, with air, and exposure of personnel to radiation.
In the past this has been accomplished with the use of rotating plugs or shields which serve to close off and seal the reactor vessel during normal reactor operation. The rotating plug systems of the prior art, of which U.S. Pat. No. 3,054,741 of J. Tatlock et al is one such example, generally included a first large plug which is approximately the size of the reactor vessel, and which may or may not also be substantially larger than the size of the nuclear core, and a second, smaller plug eccentrically supported thereon and having a lateral dimension approximately equal to 1/2 or greater than the lateral dimension of the core. The second plug has an eccentric opening and supports additional rotating plug so that access to every region of the core can be obtained by proper rotation of the plugs.
In the design of fast spectrum reactors, provisions must also be made for control mechanisms and usually also for instrumentation for monitoring coolant conditions around and in each fuel assembly. Practically, for safety considerations and for ease in manufacturing, the control mechanism and instrumentation leads must pass through the vessel head. Also, as can be appreciated, it is desirable that the control mechanism be situated in a symmetrical arrangement about the center of the core so as to be better able to control the reactor during normal operation and/or to be able to rapidly shut down the nuclear chain reaction therein in the event of an emergency.
In the prior art systems having relatively large rotating plug systems, it has been difficult to achieve the desirable arrangement of having control mechanism mounted on and passing through the reactor vessel head while having the control mechanisms situated in regular symmetrical pattern. This has been due to the fact that large eccentric tracks for large rotating plugs tend to, of necessity, yield nonsymmetrical arrangements of control mechanisms. Such schemes are further complicated by the desirability of providing instrumentation of the majority of fuel assemblies from in the nuclear core.
In U.S. application Ser. No. 537,216 entitled "Core Access System for Nuclear Reactors" by Dupen filed Dec. 30, 1974, a nuclear reactor arrangement was disclosed which provided through the head refueling and also a regular symmetrical pattern of control and instrumentation mechanisms. In that arrangement, there is provided a large rotating cover having a lateral dimension at least as large as the core and a plurality of small rotating covers eccentrically disposed in openings in the large cover. The invention of that application closely revolved about the size of the circular openings for the small rotating covers. The nuclear core of the reactor arrangement was comprised of a plurality of hexagonally shaped assemblies and the lateral dimension of the openings thus related to the lateral dimension of groups of these assemblies. These groups of assemblies were designated potential control clusters each of which was defined as a regular pattern of one central assembly plus a multiple of six surrounding assemblies. The potential control clusters were defined in a central portion of the core so as to provide an interfitting arrangement thereof so that every assembly in that portion of the core formed a part of one and only one potential control cluster. Then the radial dimension of each of the circular openings in which the small rotating plugs were disposed was defined to be greater than the lateral distance between the central assemblies of two adjacent and contiguous potential control clusters but less than twice that distance. Also each of the smaller rotating covers contained access openings therethrough and were provided with removable closure plugs for closing off the openings. In this way, by properly locating and selecting the number of second smaller rotating covers, every assembly of the core can be reached through the access openings upon proper rotation of the first and second rotatable covers. Also a uniform symmetrical control pattern could be maintained with the control and instrumentation mechanisms located on the reactor vessel head and positioned over the central assembly of a potential control cluster. By doing this, the means for supporting the second rotatable covers could be located between respective control mechanisms of two adjacent potential control clusters.
It is to an improved nuclear reactor arrangement which facilitates both through the head refueling and full instrumentation of the assemblies from in the nuclear core that the present invention is directed.